When the camera determines shutter speed and aperture, does it simply choose the nearest "standard" shutter speed and aperture or does it set them very precisely? E.g. if it determines that the correct aperture is f2.95 does it use f2.95 or does it use f2.8 ? My pentax cameras (film and digital) only ever display "normal" values for aperture and shutter speed. In which case you'll always be slightly under or over exposed anyway?Anthony

I think Canons are limited to 1/3 stop changes. That was a point in the marketing material for new Sony lenses. I don't know if continuous changes of aperture are very good for videos though. Better adjust the gain automagically I think.

The author claims the light reduction is a result of the inclination of light rays, which gets higher with wide apertures.

Then grab a "usual" lens, meaning no telephoto, retrofocus or pseudo-telecentric one. A classic one. Let's take a 40mm or 50mm lens on a 24x36 sensor, maximum aperture 1.4.I happen to have a 40/1.4 at home, i'll calculate on that basis, and I can even give you a graph (in French, but the only thing to know is that's real scale) :

Compute the angle of incidence of rays coming from the border of the cone for a pixel in the center. Approcimately 0.3 radians.Compute the angle of incidence of rays coming to the border of the sensor when you are at f/22 (meaning the ray from the center of the cone). Approximately 0.55 radians.

Meaning IF there are some effects from inclination of rays on the center of the frame with large aperture lenses, those are really weak in comparison to effets on the border of the sensor whatever aperture you use.(Don't imagine that sensors are made in a way which corrects that, apart from Leica M9 that's not the case).

And this is a very gentle inclination, try with some ultra-wide angle lens that isn't pseudo-telecentric and you'll see CRAZY inclination that even a f/0.5 lense won't provide.

Taking into account that every camera ends in a different exposure in the RAW file (in some cases differing by more than 1 or even 2 stops), for the same scene, metering, lens, shutter, aperture and ISO, I wonder why should be interesting finding out what camera vendors are doing to compensate for loss of light in the lenses by raising their ISO.

It's no problem at all, just take your sensor+lens, compare it to any other candidate, and choose the one performing best (noise) for your application. It's the only thing that really matters, not how accurate are the ISO values reported by the manufacturer, that are just a reference. We could in fact call them: first ISO, second ISO, third ISO,... without referring to any particular numbering, and nothing would change.

Much more interesting IMO is the DOF question. If a lens of a given maximum aperture is NOT producing the desired shallow DOF according to its f-number, then those manufacturers selling these lenses are cheating the buyer.

... grab a "usual" lens, meaning no telephoto, retrofocus or pseudo-telecentric one. A classic one. Let's take a 40mm or 50mm lens on a 24x36 sensor, maximum aperture 1.4.

Compute the angle of incidence of rays coming from the border of the cone for a pixel in the center. Approximately 0.3 radians.

Compute the angle of incidence of rays coming to the border of the sensor when you are at f/22 (meaning the ray from the center of the cone). Approximately 0.55 radians.

Meaning IF there are some effects from inclination of rays on the center of the frame with large aperture lenses, those are really weak in comparison to effets on the border of the sensor whatever aperture you use.(Don't imagine that sensors are made in a way which corrects that, apart from Leica M9 that's not the case).

True: the jargon is that even the chief ray (middle of the cone) hits the corners of the frame at a highly off-perpendicular angle if the exit pupil is too close to the sensor, as it is with "classic" wide to normal lens designs. ... and avoiding this "microlens vignetting" towards the edges of the frame is why lens design for digital cameras that have microlenses on their sensors favors keeping the exit pupil high, meaning "near-telecentric" so that wide to normal lens designs are somewhat retro-focal, not "classic" designs. And this is why wide-angle Leica rangefinder lenses in particular have troubles near the edges: the classic range-finder "symmetric" wide-angle designs have lower exit pupils than SLR lenses, which are forced by the presence of the mirror box to have a higher exit pupil. And this is turn is why Leica first kept to a smaller sensor in the M8 (avoiding the corner problem) and then adopted off-set microlenses for the M9. Meanwhile, DSLR's also started with the "smaller sensor" solution, and according to Thom Hogan, Nikon at least had been moving to more telecentric lens designs from early in the digital era, even before it launched its first full 35mm format DSLR.

And software correction for this microlens vignetting is available: it basically does for the edges and corners what Canon and Nikon are doing across the entire frame: bringing the luminosity of to the level to be expected from the chosen combination of f-stop, shutter speed, and exposure index ("ISO").

By the way:

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(Don't imagine that sensors are made in a way which corrects that, apart from Leica M9 that's not the case).

That is true in most cases, but the few medium format sensors I know of that have micro-lenses, all 44x33mm ones from Kodak, also use offset micro-lenses. See the note to Figure 6 on page 15 of http://www.kodak.com/global/plugins/acrobat/en/business/ISS/datasheet/fullframe/KAF-31600LongSpec.pdf So it seems that all medium format sensors are designed to avoid this microlens vignetting in order to accommodate all the "classic", not so telecentric medium format lenses in use: they do it by either omitting microlenses or offsetting them.

Actually, this might have changed recently, or be due to change soon: Dalsa has described a new type of microlenses that have a far wider acceptance angle and so avoid this vignetting problem, but I am not sure if any MF back on the market is using them.

Taking into account that every camera ends in a different exposure in the RAW file (in some cases differing by more than 1 or even 2 stops), for the same scene, metering, lens, shutter, aperture and ISO, I wonder why should be interesting finding out what camera vendors are doing to compensate for loss of light in the lenses by raising their ISO.

It's no problem at all, just take your sensor+lens, compare it to any other candidate, and choose the one performing best (noise) for your application. It's the only thing that really matters, not how accurate are the ISO values reported by the manufacturer, that are just a reference. We could in fact call them: first ISO, second ISO, third ISO,... without referring to any particular numbering, and nothing would change.

I agree, and I'd like to add that ISO have a definition which is independant from sensor sensibility... 100 ISO is what allows you to have a good exposition given aperture (well, T-number, more exactly) and given a certain amout of light.Doesn't matter at all if the sensor sensibility is not linear with its ISO settings.

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Much more interesting IMO is the DOF question. If a lens of a given maximum aperture is NOT producing the desired shallow DOF according to its f-number, then those manufacturers selling these lenses are cheating the buyer.

Regards

That's not an issue at all.

Take a lense that your reflex doesn't know (put some adhesive tape on your lens contacts ?), take a picture with a digital reflex and then with a film reflex, at f/8 -at least 2 stops smaller aperture than wide open to avoid inherent vigneting from inside obstruction-.You'll see the amount of vignetting is virtually the same.Yes, there might be some difference, maybe 10% because of the effect the author presents, or because film doesn't absorb light as easily if rays are highly inclined. But it will be nowhere near 1 EV.

And then remember what I said earlier : inclination of rays coming from the border of the cone with a large aperture lens is very small compared to inclination of rays that hit the border of the sensor, even at f/8.So the effect on DoF is just negligible.

True: the jargon is that even the chief ray (middle of the cone) hits the corners of the frame at a highly off-perpendicular angle if the exit pupil is too close to the sensor, as it is with "classic" wide to normal lens designs. ... and avoiding this "microlens vignetting" towards the edges of the frame is why lens design for digital cameras that have microlenses on their sensors favors keeping the exit pupil high, meaning "near-telecentric" so that wide to normal lens designs are somewhat retro-focal, not "classic" designs. And this is why wide-angle Leica rangefinder lenses in particular have troubles near the edges: the classic range-finder "symmetric" wide-angle designs have lower exit pupils than SLR lenses, which are forced by the presence of the mirror box to have a higher exit pupil. And this is turn is why Leica first kept to a smaller sensor in the M8 (avoiding the corner problem) and then adopted off-set microlenses for the M9. Meanwhile, DSLR's also started with the "smaller sensor" solution, and according to Thom Hogan, Nikon at least had been moving to more telecentric lens designs from early in the digital era, even before it launched its first full 35mm format DSLR.

And software correction for this microlens vignetting is available: it basically does for the edges and corners what Canon and Nikon are doing across the entire frame: bringing the luminosity of to the level to be expected from the chosen combination of f-stop, shutter speed, and exposure index ("ISO").

Thanks for the jargon

As I said above, software correction is not that strong. I use RF lenses on my Nex -no software correction-, and I don't get awful vignetting, even with 0.35 radians inclination of chief ray at corners (and 0.6 radians angle from extreme ray of the cone) with a 40/1.4. It doesn't have any noticeable vignetting at f/8 -I'd dare to say I'll notice a 1/3 EV vignetting.

So, even with a cone whose chief ray inclination is 0.35 radians (higher than any f/1.4 lens can provide - apart from anti-telecentric ones ? ^^), the effect is small.Software correction is useful for lenses vignetting, not "sensor vignetting", apart from ultra-wide non-telecentric lenses, at least.

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By the way:That is true in most cases, but the few medium format sensors I know of that have micro-lenses, all 44x33mm ones from Kodak, also use offset micro-lenses. See the note to Figure 6 on page 15 of http://www.kodak.com/global/plugins/acrobat/en/business/ISS/datasheet/fullframe/KAF-31600LongSpec.pdf So it seems that all medium format sensors are designed to avoid this microlens vignetting in order to accommodate all the "classic", not so telecentric medium format lenses in use: they do it by either omitting microlenses or offsetting them.

Actually, this might have changed recently, or be due to change soon: Dalsa has described a new type of microlenses that have a far wider acceptance angle and so avoid this vignetting problem, but I am not sure if any MF back on the market is using them.

Thanks for the medium format insight, I was just speaking of 24x36 and smaller, I think was right for these ?

I do not disagree that very high incidence angles are a huge problem with wide angle non telecentric lenses, the point I wanted to raise is that inclination from the border of the cone with wide aperture lense -subject of this article- is really low compared to inclination on the corners of the sensor even with (non telecentric) normal focal at whatever aperture you want.

Anyone can see that a normal non-telecentric lense doesn't have a dramatic vignetting in the corners at say f/8 -even without software correction-, so the issue cannot be that important with fast lenses...

Edit : we (they - the manufacturers) began to think a lot about ray inclination in corners with 24 or wider mm lenses on M9, or wide angle and extreme shift/tilt/whatever with view cameras.

24mm on a M9 means corners receive rays at a 0.75 radian inclination... 15mm (the lenses which makes the real big problem rise with the M9) mean 0.97 radians !!!

To get a 0.97 radians incidence angle from a ray hitting the center of the sensor, you'll have to use a f/0.33 aperture lense... (f/0.53 for a 0.75 radian angle).

Much more interesting IMO is the DOF question. If a lens of a given maximum aperture is NOT producing the desired shallow DOF according to its f-number, then those manufacturers selling these lenses are cheating the buyer.

Regards

Sorry, I don't think anybody is cheating anybody else. No lens manufacturer sells lenses by dof, but by max aperture, which is a optical constant that is simply defined by the focal length devided by the pupil opening. You can't blame lens manufacturers that these lenses might have (in my mind very slightly different) dof or bokeh depending if they're used on film or a digital sensor.

In my opinion the assumption made here is a lot better than the one on the luminous landscape article.I believe this exposure compensation specific to fast lenses only compensates for their own huge vignetting issues (from inner lense ray obstruction, the same that causes swirling bokeh), not for some sensor vignetting issue.

Could very well explain the exposure compensation, but not those first two DxO graphs. I'd like to have them with various lenses and various apertures.

Meaning IF there are some effects from inclination of rays on the center of the frame with large aperture lenses, those are really weak in comparison to effets on the border of the sensor whatever aperture you use.(Don't imagine that sensors are made in a way which corrects that, apart from Leica M9 that's not the case).And this is a very gentle inclination, try with some ultra-wide angle lens that isn't pseudo-telecentric and you'll see CRAZY inclination that even a f/0.5 lense won't provide.

Of course, nobody is saying this center 'sensel-vignetting' is nearly as bad as the wide-angle vignetting. The former is according to the data about between 1/3 and 2/3 of a stop. Vignetting numbers for some wide-angle lenses has been measured as high as 3 EV:http://www.photozone.de/canon_eos_ff/506-zeiss18f35eosff?start=1

Caring about rays inclination on f/1.2 lenses means caring about 0.35 radian angles.Corner rays from a typical 40mm lense at f/11 have a 0.35 radians inclination on an APSC sensor.Do you see any vignetting at f/11 with a 40mm lense on an APSC sensor ? No. Even without software correction.NB : the 40mm lense is supposed to be non-telecentric.

You might have a point, but I think you are making a false assumption which overestimates the corner "pixel vignetting": that there are "totally non-telecentric" 40mm SLR lenses with exit pupil height equal to that focal length of 40mm. That is not the way 35mm format SLR lenses work; certainly not Nikon F-mount ones because the flange is about 47mm from the focal plane, and so the exit pupil is higher than that: 55mm and up usually. See this table from 2003, and note that there has been a trend since then toward higher exit pupils: http://www.swissarmyfork.com/lens_table_1.htm

An quick evaluation (EOS 1Ds3 + EF 85mm f/1.2 L II), using IRIS software to study the Raw data before demosaicing in linear gamma space, indeed suggests a minute increase in read-noise at apertures wider than f/2. Whether this is also true for other lenses remains to be seen.

I am a sucker for useless tests ;-0: same result here on the 85 1.2 and 50 1.4 on the 5DMKII

(4 secs dark frames at 400 ISO, separated by 20 seconds to let the sensor cool, in a dark room to avoid light leaks, analyzed in Iris with bgnoise, average of 2 runs, confirmed with Maxim)

Your noise statistics for the 5D2 show similar values as for my 1Ds3, although you took 4 second dark frames at ISO 400, while I took 1/8000 second black frames at ISO 100. That only confirms the potentially improved output from the 5D2 (more recent design), if it were not for the higher pattern noise probability.

You also experienced the slight increase of gain generated noise at apertures wider than f/2.0. So that kind of confirms that something is going on, although the increase of noise is very slight. Because the gain boost is so small, and assuming it is to compensate for exposure gate shading (QED), I do wonder why the manufacturers even do it, because the effects of vignetting plus light fall-off towards the corners of the image are much more pronounced, and spatially variant. The larger issue at wide aperture, as far as exposure is concerned, is the significant under exposure of the corners of the image, which leeds to visible differences in signal to noise after vignetting correction in postprocessing.

Another, somewhat related, issue is 'color cast'. That's a shift in color balance which also depends on the angle of incidence, but that is also not addressed by an overal minor gain boost.

The reason for the gain boost remains slightly puzzling, but given it's minor effect on noise (and only in scenarios that create larger problems), it's not a big issue. For those who like to understand their tools, it's intriguing though ...

Of course, nobody is saying this center 'sensel-vignetting' is nearly as bad as the wide-angle vignetting. The former is according to the data about between 1/3 and 2/3 of a stop. Vignetting numbers for some wide-angle lenses has been measured as high as 3 EV:http://www.photozone.de/canon_eos_ff/506-zeiss18f35eosff?start=1

You might have a point, but I think you are making a false assumption which overestimates the corner "pixel vignetting": that there are "totally non-telecentric" 40mm SLR lenses with exit pupil height equal to that focal length of 40mm. That is not the way 35mm format SLR lenses work; certainly not Nikon F-mount ones because the flange is about 47mm from the focal plane, and so the exit pupil is higher than that: 55mm and up usually. See this table from 2003, and note that there has been a trend since then toward higher exit pupils: http://www.swissarmyfork.com/lens_table_1.htm

Unless I have rangefinder lenses -not telecentric- and I use them on my APSC Nex ? Which is exactly what I do :-) And I happen to have a 40mm I measured myself yesterday.

I also have a 50/2.8 Sony macro lense, whose exit pupil is somewhere like 5mm from flange. Sony has a 45mm-ish flange.And DxO says its lens have a mere 1/3 EV sensor vignetting on a full frame sensor. So basically DxO says that 0.40 radians inclination doesn't mean more than 1/3EV sensor vignetting (and it might even be the cos^4 law talking and not pixel vignetting).

You can test it yourself by grabbing any lens in the list you gave (thanks, it's really intersting by the way ) that have a 55mm (from sensor) exit pupil : you'll have a 0.37 radians angle which correspond to a 1.2 aperture.Do you have awful vignetting in corners (on FF sensors) with those lenses once closed to f/11 ? Probably not.

I don't have it on my Nex with a 40mm non-telecentric lens, DxO doesn't measure it stronger than 1/3 EV with a non-telecentric 50mm on FF sensor...... and friends of mine start to have that kind of vignetting and magenta shift with 20mm -non telecentric- lenses and shorter : angles like from f/0.7 lenses...

Another, somewhat related, issue is 'color cast'. That's a shift in color balance which also depends on the angle of incidence, but that is also not addressed by an overal minor gain boost.

The reason for the gain boost remains slightly puzzling, but given it's minor effect on noise (and only in scenarios that create larger problems), it's not a big issue. For those who like to understand their tools, it's intriguing though ...

Color cast happens on APSC sensors with 20mm exit pupils and shorter, meaning angles like 0.6 radians, such as with f/0.7 lenses... Nothing to worry about at f/1.2 or on corners with DSLR lenses.

I firmly believe this gain boost is here to compensate for LENS vigneting (mainly optical vigneting) which is often severe with wide aperture lenses all over the frame apart from center -look at that swirling bokeh-, and give images an overall under-exposed look.If you know your f/1.4 lens has a -2.5 ev vignetting in corners, -1.5 ev on horizontal borders, and -1 EV overall... A 1 EV overexposure will over-expose the center -what DxO measured, I think-, but the whole image will look better...

I firmly believe this gain boost is here to compensate for LENS vigneting (mainly optical vigneting) which is often severe with wide aperture lenses all over the frame apart from center -look at that swirling bokeh-, and give images an overall under-exposed look.If you know your f/1.4 lens has a -2.5 ev vignetting in corners, -1.5 ev on horizontal borders, and -1 EV overall... A 1 EV overexposure will over-expose the center -what DxO measured, I think-, but the whole image will look better...

I'm not so sure. The gain boost I'm getting seems to be in the order of a fraction of 1/3rd of a stop (I need to do a more exact determination, but IRIS is not cooperating). No way is that going to improve the loss of 1 or more stops in the corners in any significant manner. When using the internal exposure meter, the automatic correction for corner underexposure may well turn out to be more by adding actual photons, but overexposure of the image center (assuming I'm 'exposing to the right' there, e.g. with manual exposure) is not going to help image quality either. The puzzle remains.

The reason for the gain boost remains slightly puzzling, but given it's minor effect on noise (and only in scenarios that create larger problems), it's not a big issue. For those who like to understand their tools, it's intriguing though ...

I agree, Bart. The effect is interesting and we should thank Mark Dubovoy for raising the issue. But I wonder just how useful in a pratical sense in the field or studio such knowledge would be if the manufacturer were to specify and reveal any current secret boosting of ISO when a lens is used at very wide apertures.

We really need some comparison shots demonstrating that an F1.2 shot with a particular lens really has no shallower DoF, and/or has noticeably greater noise, than an F1.4 shot of the identical scene using the same lens.

If the increase in noise (or lack of reduction in DoF) is not noticeable, except at the extreme pixel-peeping level, then the issue becomes academic and perhaps only of concern to organizations like DXO Labs who need extremely accurate information on the performance of sensors and lenses in order to improve their RAW converter.

Another issue with such a comparison is the resolution at the point of focus. We would expect an F1.2 lens to be slightly sharper at F1.4 and slight sharper again at F1.8.If we have a situation whereby a shot at F1.4 has a noticeably blurrier background than the same scene shot at F1.8, but the plane of focus in the F1.8 shot is noticeably sharper, then at a certain print size which brings out that greater sharpness, the differences in the perceived DoF of the two shots will be reduced.

What is perhaps of greater concern to the practical photographer is the variance in 'light transmission' efficiency of different lenses used at the same f stop. Do we need T-stop markings on lenses? Is the variance great enough to warrant that?

The outcome was, I learned that my Canon 50/1.8 has about 2/3rds of a stop greater light-transmission efficiency than my Canon 24-105/F4 zoom at 50mm. I also bought a Nikon D700 and sold my Mark Welsh adapter.

The practical benefit of such knowledge might be of some significance in certain circumstances when, for example, I need a 50mm lens and a fast shutter speed for a moving subject. I might not bother changing lenses from the zoom to the 50mm prime for the sake of the marginal increase in resolution that the 50mm prime might provide.

However, knowing that I can also use a faster shutter speed at the same ISO and f stop (up to F4) with the 50/1.8 (a 320th as opposed to a 200th) for the same ETTR exposure, I might just take the trouble to change lenses and get a better quality shot as a consequence.

By the way, the Nikkor 14-24 on the D700 seems to have the same T-stop as the Sigma 15-30 on the 5D. The variance is very slight and appears to match the very slight differences in ISO sensitivity between these two cameras as shown on the DXOmark site.

The issue is NOT the way the sensor is built. This is easy to prove by using the tables Mark provided in his argument. Cameras with essentially identical sensors, but different sensor sizes have completely different results when the lens is used at F1.2.

The reason is NOT the sensor!

It's the mirror box. If you look at the exit pupil of the lens in question you will see that it is actually larger than the mirror box itself. The mirror box itself is vignetting the light path. The tighter the mirror box, the more vignetting you get.

Those of us who have a bit of experience with Perspective Control lenses have run into this problem for ages. We used to blame the problem on lens falloff, but it was actually light-loss due to the walls of the mirror-box impinging on the light path.

I think some posters (BartvanderWolf, pegelli, b_z and image66 among others) haven't understood Marc Dubovoy's excellent article because they are confused about what microlenses accomplish.

Microlenses improve the angular response of a sensel, but only up to a limit.

The marginal rays of the light cone projected by a fast — e.g. f/1.2 — lens are quite tilted.

A pont source of light, e.g. a small LED photographed from a fairly large distance, stars in the night sky etc. form, when in focus, a point on the imaging sensor or film.

When out of focus, the intersection between the lens' light cone and the imaging plane forms a disc, not a point.Depending on the degree of defocus, that disc can obviously have various diameters.If the degree of defocus is small, the disc diameter might be smaller than the diameter of the circle of confusion which figures in depth of field calculations.Regardless of the OOF disc's diameter, the angle of the marginal rays relative to the sensor doesn't change. This obviously implies, as Mark Dubovoy points out, that the diameter of the circle of confusion recorded by an imaging sensor is also affected by the sensel's acceptance angle, and that depth of field, in turn, must be affected by the sensel's angular response.

How can we estimate that acceptance angle with any degree of reliability ?

If, despite the microlens' best efforts, the marginal rays are too tilted to reach the photodiode at the bottom of thick sensel structure, these light rays can be considered to be non-existent for imaging purposes.

If these marginal rays cannot reach the photodiodes, the recorded diameter of the disc formed by an out of focus point light source will be reduced.

A simple way to assess the critical incidence angle — that is, the acceptance angle — above which the rays cannot be recorded anymore is thus to measure the diameter of the out of focus disc formed by a point source.

Film doesn't have any problems recording even very tilted light rays. Due to the very geometry of the lens' light cone, the diameter of the OOF disc must be directly proportional to the lens' f-stop a.k.a. aperture setting. On film, doubling the aperture thus necessarily doubles the recorded OOF disc diameter.

With some digital sensor designs, acceptance angles can be quite limited. At some point, the linear relationship between the lens aperture and the OOF disc diameter recorded on the picture must then break down, as the tilted marginal rays cannot reach the photodiodes anymore.

A quick measurement of the dimensions of the OOF discs visible in the test picture of the Canon EF50mm F/1.2L lens on Photozone.de show that with the Canon 5D Mark II's CMOS sensor used in the test, the diameter of the OOF disc increases, as expected, when the lens is opened up from f/4 to f/2, but then plateaus at a diameter geometrically corresponding to about f/1.5, even when the lens is opened up to f/1.2

This indicates that the acceptance angle of the Canon 5D Mark II's sensor is limited to about arctan(1/(1.5*2)) i.e. about 18.4 degrees from the perpendicular, and that a f/1.2 lens thus performs on the 5D2 essentially as a f/1.5 lens as far as actual lens speed, DoF and bokeh are concerned.To fully record the bokeh and "draw" of fast lenses like the Canon EF50mmF1.0L, EF50mmF1.2L and EF85mmF1.2L, ir thus seems that one will have to use a film-based Canon EOS body, instead of a Canon DSLR.

An interesting pint of comparison to bring up here might be the — presumably identical — sensels used in the CCD sensors equipping the Leica M8 and M9.CCDs don't need the multiple transistors surrounding each photodiode of a CMOS sensor architecture. As such, CCDs don't require the multiple metal layers of a CMOS needed for the transistor's signal lines. The resulting CCD pixel stack is typically much more shallow than a CMOS sensor's tunnel-like architecture.With a CCD, the distance between the microlens and the photodiode can thus be made closer than with a CMOS sensor, and the microlens' acceptance angles can thus be much wider.

The marginal ray of the light cone of a Leica Noctilux f/1.0 lens is about arctan(1/2) = 26.6 degrees. According to its datasheet, the Kodak KAF-10500 CCD used in the Leica M8 still has an angular response, at an angle of 27 degrees, of about 70% of the peak response.Unlike the DSLRs, the CCD-based digital Leicas thus seem entirely able to record the light rays of the wide light cone emanating from Leica's super-fast Noctiluxes.

As for b_z's assertions about acceptance angles, using his NEX-5 and a 40mm F/1.4 lens as examples, they are flawed for several reasons:

b_z doesn't properly consider the distance between the sensor and the lens' exit pupil, from which the light rays can be considered to emanate. That exit pupil distance is unknown. If it is 40mm, the chief ray's tilt angle, even in the extreme corner of an APS-C sensor, would be arctan(14mm/40mm) i.e. about 19.2 degrees, that is, quite close to the Canon 5D2's f/1.5-equivalent 18.5 degrees. As such, given the incertitude about the exit pupil distance of the 40mm lens used by b_z, his test doesn't bring any useful piece of information about the angular response of f/1.2 lenses.

The minimal exit pupil distance relative to the image plane of most Leica-mount rangefinder lenses, including ultra wides, is about 28mm. I know of no actual instances of Leica-mount lenses with a 15mm exit pupil distance hypothesized by b_z.

The Sony-brand APS sensor used in Fuji's X100 fixed-lens digital rangefinder camera uses offset microlenses. There is no publicly available information indicating whether Sony uses offset microlenses for the NEX series of cameras. This means that it is impossible for us to state whether the NEX-5 uses, or does not use offset microlenses. If the NEX-5 uses offset microlenses — a plausible scenario, given the very short flangeback of the NEX mount and the compactness of the NEX 16mm pancake lens — the acceptance angle of the corner sensels could be even larger than the back-of-the envelope calculations done above, rendering b_z's NEX-based analysis even more meaningless.

Let us now consider image66's assertion that the vignetting is not caused by the sensor, but by the mirror box.

If the mirror box were to cause vignetting at the center of the image where DxO performed their mesurements, then, the OOF disc of an f/1.2 or f/1.4 lens at the center of the image would not appear as a perfect circle, but as a circle visibly truncated by the straight edges of the mirror box.

Given that anybody with an APS or full-size DSLR and a f/1.2 or f/1.4 lens can verify that, even with the lens wide open, an OOF point lioght source appears at the center of the image as a perfect circle and not as a truncated one, image66's assertion can be dismissed out of hand.

As a camera geek, I applaud Mark Dubovoy for his very insightful article, and look forward to DxO publising their detailed findings as to how they assessed the amplitude of the ISO correction camera manufacturers automatically apply to compensate for the non-recording of the marginal light rays emitted by fast lenses.

I think some posters (BartvanderWolf, pegelli, b_z and image66 among others) haven't understood Marc Dubovoy's excellent article because they are confused about what microlenses accomplish.

Mark's article is good because it's a relevant question to ask (regarding false ISO setting), but it's not excellent because his ideas of a cause and his whole "depth of field" idea is totally irrelevant here.

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Microlenses improve the angular response of a sensel, but only up to a limit.

Yes.

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The marginal rays of the light cone projected by a fast — e.g. f/1.2 — lens are quite tilted.

A LOT less than rays coming to corners of sensor with a telemetric (non telecentric) wide angle. And still less than a ray coming to corner of a FF sensor with a 50mm non telecentric macro lense. See my calculation above, appearently you can understand them.

A pont source of light, e.g. a small LED photographed from a fairly large distance, stars in the night sky etc. form, when in focus, a point on the imaging sensor or film.

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When out of focus, the intersection between the lens' light cone and the imaging plane forms a disc, not a point.Depending on the degree of defocus, that disc can obviously have various diameters.If the degree of defocus is small, the disc diameter might be smaller than the diameter of the circle of confusion which figures in depth of field calculations.Regardless of the OOF disc's diameter, the angle of the marginal rays relative to the sensor doesn't change.

True

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This obviously implies, as Mark Dubovoy points out, that the diameter of the circle of confusion recorded by an imaging sensor is also affected by the sensel's acceptance angle, and that depth of field, in turn, must be affected by the sensel's angular response.

Yes, ONLY IN THE MIDDLE OF THE SENSOR, borders of the circle consists of inclinated rays. But as soon as you're not in the center anymore, that's NOT TRUE. And it does not appear in reality.

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How can we estimate that acceptance angle with any degree of reliability ?

Reading my calculations above would be a good start...

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If, despite the microlens' best efforts, the marginal rays are too tilted to reach the photodiode at the bottom of thick sensel structure, these light rays can be considered to be non-existent for imaging purposes.

Not, the effect is progressive.

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If these marginal rays cannot reach the photodiodes, the recorded diameter of the disc formed by an out of focus point light source will be reduced.

No, the effect is progressive. At most you should see border of the circle in the middle of the frame fade a little, nothing more.

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A simple way to assess the critical incidence angle — that is, the acceptance angle — above which the rays cannot be recorded anymore is thus to measure the diameter of the out of focus disc formed by a point source.

Not true, see above.

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Film doesn't have any problems recording even very tilted light rays. Due to the very geometry of the lens' light cone, the diameter of the OOF disc must be directly proportional to the lens' f-stop a.k.a. aperture setting. On film, doubling the aperture thus necessarily doubles the recorded OOF disc diameter.

True

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With some digital sensor designs, acceptance angles can be quite limited. At some point, the linear relationship between the lens aperture and the OOF disc diameter recorded on the picture must then break down, as the tilted marginal rays cannot reach the photodiodes anymore.

False, the effect is PROGRESSIVE.

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A quick measurement of the dimensions of the OOF discs visible in the test picture of the Canon EF50mm F/1.2L lens on Photozone.de show that with the Canon 5D Mark II's CMOS sensor used in the test, the diameter of the OOF disc increases, as expected, when the lens is opened up from f/4 to f/2, but then plateaus at a diameter geometrically corresponding to about f/1.5, even when the lens is opened up to f/1.2

Absolutely irrelevant example, what you're seeing there is that because the disc is NOT IN THE MIDDLE OF THE FRAME, the lens show mechanical vignetting because of the conception of the lens. It's related to vigneting, and bokeh discs into sort-of-elliptic shapes.You're just seing that, nothing else.

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This indicates that the acceptance angle of the Canon 5D Mark II's sensor is limited to about arctan(1/(1.5*2)) i.e. about 18.4 degrees from the perpendicular, and that a f/1.2 lens thus performs on the 5D2 essentially as a f/1.5 lens as far as actual lens speed, DoF and bokeh are concerned.To fully record the bokeh and "draw" of fast lenses like the Canon EF50mmF1.0L, EF50mmF1.2L and EF85mmF1.2L, ir thus seems that one will have to use a film-based Canon EOS body, instead of a Canon DSLR.

Not at all, try on film you'll see exactly the same shape. It's due to lens design, not sensor.

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An interesting pint of comparison to bring up here might be the — presumably identical — sensels used in the CCD sensors equipping the Leica M8 and M9.

M8 is APSC and have no telecentric microlenses. The sensors are TOTALLY different in regard to their response to rays inclination.

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CCDs don't need the multiple transistors surrounding each photodiode of a CMOS sensor architecture. As such, CCDs don't require the multiple metal layers of a CMOS needed for the transistor's signal lines. The resulting CCD pixel stack is typically much more shallow than a CMOS sensor's tunnel-like architecture.With a CCD, the distance between the microlens and the photodiode can thus be made closer than with a CMOS sensor, and the microlens' acceptance angles can thus be much wider.

Well, I'm not sure about that, and the fact that DxO datas show that CCDs are WORSE in regard to this "unknown" effect makes this assumption quite weird.

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The marginal ray of the light cone of a Leica Noctilux f/1.0 lens is about arctan(1/2) = 26.6 degrees. According to its datasheet, the Kodak KAF-10500 CCD used in the Leica M8 still has an angular response, at an angle of 27 degrees, of about 70% of the peak response.Unlike the DSLRs, the CCD-based digital Leicas thus seem entirely able to record the light rays of the wide light cone emanating from Leica's super-fast Noctiluxes.

What's the angular response for say the A900 CMOS ? You don't know ? Then how could you compare

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As for b_z's assertions about acceptance angles, using his NEX-5 and a 40mm F/1.4 lens as examples, they are flawed for several reasons:

It's rather funny you call my assumptions flawed when in fact you just didn't understand them, or considered I hadn't thought of testing some things.

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b_z doesn't properly consider the distance between the sensor and the lens' exit pupil, from which the light rays can be considered to emanate. That exit pupil distance is unknown.

As a matter of fact, I consider lens's exit pupil.

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If it is 40mm, the chief ray's tilt angle, even in the extreme corner of an APS-C sensor, would be arctan(14mm/40mm) i.e. about 19.2 degrees, that is, quite close to the Canon 5D2's f/1.5-equivalent 18.5 degrees. As such, given the incertitude about the exit pupil distance of the 40mm lens used by b_z, his test doesn't bring any useful piece of information about the angular response of f/1.2 lenses.

Exit pupil of my 40mm lens is somwhere between 35 and 45mm. I measured that. Now, you see why my corner rays are relevant ? They are more tilted than this f/1.2 lens rays. And still I got no real vignetting when stopped down a little.

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The minimal exit pupil distance relative to the image plane of most Leica-mount rangefinder lenses, including ultra wides, is about 28mm. I know of no actual instances of Leica-mount lenses with a 15mm exit pupil distance hypothesized by b_z.

Look at voigtlander 15mm. Just look.

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The Sony-brand APS sensor used in Fuji's X100 fixed-lens digital rangefinder camera uses offset microlenses. There is no publicly available information indicating whether Sony uses offset microlenses for the NEX series of cameras. This means that it is impossible for us to state whether the NEX-5 uses, or does not use offset microlenses. If the NEX-5 uses offset microlenses — a plausible scenario, given the very short flangeback of the NEX mount and the compactness of the NEX 16mm pancake lens — the acceptance angle of the corner sensels could be even larger than the back-of-the envelope calculations done above, rendering b_z's NEX-based analysis even more meaningless.

You don't know if Nex sensor has offset microlenses. I do know it doesn't have any. Saying my analysis is meaningless just because you don't understand and don't even try to search on your own to check my sayings is quite rude.

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Let us now consider image66's assertion that the vignetting is not caused by the sensor, but by the mirror box.

If the mirror box were to cause vignetting at the center of the image where DxO performed their mesurements, then, the OOF disc of an f/1.2 or f/1.4 lens at the center of the image would not appear as a perfect circle, but as a circle visibly truncated by the straight edges of the mirror box.

Given that anybody with an APS or full-size DSLR and a f/1.2 or f/1.4 lens can verify that, even with the lens wide open, an OOF point lioght source appears at the center of the image as a perfect circle and not as a truncated one, image66's assertion can be dismissed out of hand.

As a camera geek, I applaud Mark Dubovoy for his very insightful article, and look forward to DxO publising their detailed findings as to how they assessed the amplitude of the ISO correction camera manufacturers automatically apply to compensate for the non-recording of the marginal light rays emitted by fast lenses.

Mark's questions are good, but his (and your) assumptions about the causes are wrong.

This ISO correction could be made only to compensate the huge mechanical vignetting every fast lens showand make the whole image look better exposed.